Why do I need to map IntStream to Stream<Character>

user_3981312 0
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  public static int construction(String myString) {
      Set<Character> set = new HashSet<>();

      int count = myString.chars()  // returns IntStream
      .mapToObj(c -> (char)c)       // Stream<Character> why is this required?
      .mapToInt(c -> (set.add(c) == true ? 1 : 0)) // IntStream
      .sum();

      return count;
    }

The above code will not compile without: 

.mapObj(c -> (char)c)
// <Character> Stream<Character> java.util.stream.IntStream.mapToObj(IntFunction<? extends Character> mapper)

If i remove it, I get the following error

The method mapToInt((<no type> c) -> {}) is undefined for the type IntStream

Can someone explain this? It seems like I am starting with and IntStream, converting to a Stream of Characters and then back to IntStream.

java-stream java-8 set collections java
3 Answers

The method CharSequence::chars returns the IntStream, which of course doesn't provide any method converting to int, such as mapToInt, but mapToObj instead. Therefore the method IntStream::map(IntUnaryOperator mapper) which both takes returns int as well shall be used since IntUnaryOperator does the same like Function<Integer, Integer> or UnaryOperator<Integer>:

int count = myString.chars()                 // IntStream
    .map(c -> (set.add((char) c) ? 1 : 0))   // IntStream
    .sum();

long count = myString.chars()                // IntStream
    .filter(c -> set.add((char) c))          // IntStream
    .count();

Also, using Set<Integer> helps you to avoid conversion to a Character:

Set<Integer> set = new HashSet<>();

int count = myString.chars()                 // IntStream
    .map(c -> (set.add(c) ? 1 : 0))          // IntStream
    .sum();

long count = myString.chars()                // IntStream
    .filter(set::add)                        // IntStream
    .count();

However, regardless of what you try to achieve, your code is wrong by principle. See the Stateless behaviors. Consider using the following snippet which lambda expressions' results are not dependent on the result of a non-deterministic operation, such as Set::add.

Stream pipeline results may be nondeterministic or incorrect if the behavioral parameters to the stream operations are stateful.

long count = myString.chars()             // IntStream
                     .distinct()          // IntStream
                     .count();
user_37649650

You can also collect to a set and then take the size without using an explicit map. It does not require using external state to contain the characters.

    long count = str.chars().boxed().collect(Collectors.toSet()).size();

But imho, the more direct approach which was already mentioned is cleaner in appearance and the one I would prefer to use.

    long count = str.chars().distinct().count();
user_15525340

Because String.chars() is already returning an IntStream and IntStream does not have mapToInt function

You could use a filter instead then count:

int count = myString.chars()
      .filter(c -> set.add(c) == true)
      .count();

I admit that I made this so slubby last midnight! As mentioned by the comments, here is the required fixes.

Thank you for mentioning.

long count = myString.chars()
          .filter(c -> set.add((char)c))
          .count();
user_25563540
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